As a method for recording digital data on an optical disc as an information storage medium, there has been commonly employed a method for uniformizing the recording density on a recording medium by making the linear velocity constant, as seen in a compact disc (registered trademark) (hereinafter referred to as a CD) and a digital versatile disc (hereinafter referred to as a DVD).
Conventionally, when reading digital data from an optical disc on which data are digitally recorded by performing mark width modulation so as to make the linear recording density constant, the phase of a clock component corresponding to a channel bit frequency possessed by the read signal is detected to constituted a phase-locked loop, thereby performing phase-locked pull-in. Further, in order to improve playback performance of high-density recordable media such as a DVD-RAM (DVD-Random Access Memory) and a BD (Blu-ray Disc), there has also been introduced a method of performing demodulation of digital data by using a PRML (Partial Response Maximum Likelihood) signal processing method that is effective to high-density recording/playback in the linear direction.
There has conventionally been a disc playback system shown in FIG. 17, as a system that enables the phase-locked pull-in, and realizes a digital signal processing method such as the PRML signal processing method.
In the conventional disc playback system, digital codes (NRZI (Non Return to Zero Invert) codes) as shown in the upper stage in FIG. 18 are recorded so that the linear recording density becomes constant. It is premised that the recorded data are data in which the number of continuous 0s or 1s is within a range from 3 to 14, like the 8-16 modulation method. An optical disc playback signal 73 that is obtained by playing the disc with an optical playback means 2 such as an optical pickup is attenuated in amplitude as the frequency band becomes higher due to interference with an increase in the recording density in the linear direction of the recorded data. So, the signal 73 is amplified by a preamplifier 3 and then subjected to correction so as to emphasize the high-frequency component by using a waveform equalization means 4, as shown in the upper stage in FIG. 18.
The high-frequency emphasized playback signal is sampled to a multiple-bit playback digital signal 63 by an analog-to-digital converter (AID converter) 5 that is a means for converting an analog signal into a digital signal by using a playback clock 64 generated by a VCO (Voltage Controlled Oscillator) 62. At this time, if the phase of the playback clock 64 is synchronized with the phase of the clock component possessed by the playback signal, sampling data as shown in the middle stage and the lower stage in FIG. 18 are obtained. The middle stage in FIG. 18 shows a sampling method in which binarization judgement is carried out at an arbitrary level, and the lower stage in FIG. 18 shows a sampling method that is particularly suited to the PRML signal processing method.
The PRML signal processing method is as follows. That is, in a playback system in which the amplitude of high-frequency component is degraded with an increase in linear-direction recording density and thereby the S/N ratio is increased, waveform interference is intentionally added by adopting the partial response method to realize a playback system that needs no high-frequency component, and moreover, the quality of playback data is improved by the maximum likelihood decoding method that estimates a most likely string by probability calculation considering the waveform interference (for example, refer to Japanese Published Patent Application No. 2002-269925 (Pages 5, 6, 12-14, FIGS. 3, 10, and 27)).
Then, the multiple-bit playback digital signal 63 outputted from the A/D converter 5 is input to an offset correction means 7 to correct an offset component included in the playback digital signal, i.e., an offset in the amplitude direction from the center level at which code balance is kept. The offset-corrected playback digital signal is demodulated to a digital binary signal by a PRML signal processing means 17 comprising a transversal filter and a Viterbi decoder. At this time, by adopting the partial response equalization, the equalized output signal outputted from the transversal filter to the Viterbi decoder is multi-valued to a quinary signal in the PRML signal processing means 17 (refer to the lower stage in FIG. 18). The quinary equalized output signal is subjected to probability calculation by the Viterbi decoder to generate a binary value.
Further, the playback clock 64 to be used for performing sampling by the A/D converter 5 is generated as follows.
Initially, a phase control amount for performing phase synchronization control between the playback clock and the playback digital signal is generated by a phase synchronization control means 10 comprising a phase comparator, a loop filter, and a D/A converter, using the output signal of the offset correction means 7. The VCO 62 is controlled on the basis of the phase control amount, and the playback clock 64 is obtained as an output of the VCO 62.
Through the series of processings described above, the phase of the playback clock is synchronized with the phase of the clock component possessed by the playback digital signal, and the PRML signal processing method is adopted, whereby the digital data recorded on the optical disc can be reproduced with stability and accuracy (for example, refer to Japanese Published Patent Application No. 2000-123487 (Page 4, FIG. 9)).
Furthermore, there has been proposed a signal processing apparatus in which an optical playback waveform is converted into a digital playback signal by an A/D converter using a clock of a frequency higher than the channel bit frequency, and a digital PLL is constituted utilizing an interpolation filter in the phase direction, thereby generating a phase-locked playback digital signal to perform digital data demodulation (for example, refer to Japanese Patent No. 3255179 (page 5, FIG. 1)).
In the conventional optical disc playback apparatus constituted as described above, the playback signal is converted into a multiple-bit discrete signal by the A/D converter using the channel bit clock that is synchronized with the channel bit frequency as a clock component possessed by the playback waveform from the optical disc, and the PLL (Phase Locked Loop) is constituted using the signal to carry out phase-locked pull-in control and data demodulation such as PRML signal processing.
However, when the PLL is constituted by using the multiple-bit discrete signal after AID conversion, which is generated on the basis of the channel bit clock, detection of phase error information or the like takes time, and therefore, the delay time of the control loop increases to degrade the phase-locked pull-in performance. As a result, the phase-locked pull-in control is initially failed against degradation in quality of the playback waveform due to a tilt that is defined by an angle formed between an axis perpendicular to the recording surface of the optical disc and an axis of the incoming laser beam, or local degradation in playback characteristics depending on playback under poor S/N ratio, asymmetry of upper and lower portions of the playback waveform, or defects such as flaws, contamination, and finger prints on the disc surface. Consequently, the effectiveness of the PRML signal processing method cannot be satisfactorily utilized.
Further, in the optical disc playback apparatus employing both of the PLL and the PRML signal processing means, it is possible to obtain two kinds of binary outputs, i.e., the binary output from the PRML signal processing means, and the binary output obtained by subjecting the output of the phase comparison means in the PLL to level judgement.
The binary output, which is obtained by judging the output of the phase comparison means as “1” (“0”) when it is equal to or larger (smaller) than the level of the waveform (hereinafter referred to as a level judgement method), is resistant to flaws on the disc surface, and stable playback can be carried out even when a position having flaws is played. On the other hand, the binary output from the PRML signal processing means (hereinafter referred to as a PRML signal processing method) is resistant to the case where a DVD-R or DVD-RW having originally degraded signal quality is played, or to tilt degradation, and stable playback can be carried out even when playing the disc of low signal quality.
However, since the level judgement method and the PRML signal processing method are realized by changing the sampling method, it is impossible to realize, simultaneously and at maximum precision, the level judgement method that is stable against burst errors caused by defects or the like, and the PRML signal processing method that is effective to linear-direction high-density recording/playback. Therefore, an optimum detection method cannot be selected according to the playback condition. Further, since the information in the time direction is lost, the precision in detecting jitter information tends to deteriorate. As a result, there are cases where learning of cut-off frequency or boost amount during focus servo or waveform equalization does not converge to an optimum value due to considerable asymmetry or the like, leading to degradation in the playback performance.
On the other hand, since Japanese Patent No. 3255179 employs the over-sampling clock, it can solve the above-mentioned problem such as tilt degradation caused by the convergence speed of the PLL.
However, the over sampling method disclosed in Japanese Patent No. 3255179 is asynchronous over sampling method in which the relation between the channel bit frequency and the over sampling is not an integral multiple. The asynchronous over sampling has an advantage in that the PLL can be constituted by a synthesizer instead of a VCO whose circuit scale is relatively large. However, in the field of optical discs, with respect to DVD alone, plural kinds of optical discs have been put to practical use in recent years, and therefore, it is necessary to prepare synthesizers as many as the kinds of optical discs in order to deal with various kinds of optical disc playbacks with a single optical disc device, resulting in an increase in circuit scale. Further, the asynchronous over sampling method complicates the PRML circuit.